How to Use 0.002mm. as the Grain Size Silt/Clay Division

Several users have inquired as to how to set up their grain size test so that it uses 0.002mm. as the breakpoint between silt and clay particle sizes. There's two options for doing this; the "best" option is subject to engineering judgement and the requirements of the test you performed!

  • If you're classifying the soil via AASHTO or USDA classification methods, select either "AASHTO" or "USDA" as your grain size test's "Classification system" (select "Window" > "Sample Info." from the menu at the top of the screen, then look for the "Classification system" box towards the bottom of the screen).
  • If you're sure that you want to use the divisions specified by the USCS classification system (e.g., gravel is between 3/4" and 3", sand between the #200 and the #4 sieve, etc.), but you want the silt/clay division to be 0.002mm. instead of 0.005mm., firstly, make sure that you know why you want to do so! 0.005mm. is the appropriate silt/clay division size for the USCS/ASTM D 2487 soil classification method. We're finding that many times users ask to switch to 0.002mm. because their DOT told them to use that division: One clue -- if it's a state DOT making the request, it's because they use the AASHTO classification system's divisions (see the previous paragraph).

    If, after due diligence, you determine that you actually do need to modify the USCS classification system to use a 0.002mm. silt/clay division, do the following: Open your grain size test, select "Window" > "Sample Info." from the menu at the top of the screen, then select "USCS with .002mm. instead of .005 silt/clay division" as the test's "Classification system" (look for the box towards the bottom of the screen).